It has been desirable to transmit and receive not only the voices of parties to a telephone conversation, but also their faces and other images. Early attempts to transmit images involved sending and receiving still images using standard telephone lines. More recently, data compression technology has facilitated the transmission of moving images using standard telephone lines. These compression techniques have also been used to compress audio information, such as speech. Compressing both audio and video information allows the information to be sent within the bandwidth constraints of conventional telephone lines. As a result, it is possible to transmit and receive telephone-quality speech and video images of adequate quality.
Many conventional speech-only telephones, on the other hand, transmit speech as analog signals rather than as compressed digital signals. With videophones and speech-only telephones employing different transmission techniques, it has been difficult for videophones to communicate with speech-only telephones. As a result, use of videophones has been largely limited to communication with compatible videophones. The lack of compatibility between videophones and conventional speech-only telephones has hampered the proliferation of videophones.